Micro-display devices having LCoS structures (or, equivalently LCoS devices) are becoming increasingly prevalent in various micro-display applications, such as big-screen TVs, PC monitors, projectors, etc. Typically, an LCoS device has a semiconductor substrate and a liquid crystal positioned on the substrate, where the light passed through the liquid crystal may be magnified by a suitable optical system to display images formed on the liquid crystal for human eyes.
In general, vitally important elements to generate a good LCoS image are contrast, brightness, and resolution. Resolution may be determined by the number of pixels within an image. Currently, there is a number of resolution standards defined for various electronic applications. For example, a conventional high-definition TV (HDTV) screen may have 1,920 and 1,080 scan lines in the horizontal and vertical directions, respectively. In general, higher resolution may yield better image quality. Brightness refers to the backlight luminescence of an LCoS image. For a given contrast and resolution, the image sharpness may be enhanced by increasing the brightness of the image. Contrast or contrast ratio refers to the ratio of luminance between the brightest white that can be produced and the darkest black that can be produced. Contrast ratio is the major determinant of perceived picture quality: if an image has a high contrast ratio, viewers will judge it to be sharper than a picture with a lower contrast ratio, even if the lower contrast picture has a substantially higher resolution.
Thus, one approach to improve the image quality for an LCoS device may be increasing the resolution, i.e., increasing the number of pixels for impressing the image on the liquid crystal. In general, the size of each pixel may decrease as the resolution increases, which increases spatial proximity between two neighboring pixels and circuit elements within the LCoS device chip. The increases spatial proximity may induce an electrical noise that stems from cell-to-cell cross-talk or coupling effect between the circuit elements. In general, conventional non-LCoS semiconductor chips do not use high voltage signals and thus the electrical noise may not be significant. In contrast, a typical LCoS micro-display device chip may require high voltage signals to form images in the liquid crystal. When the high voltage signals transmit through the circuit elements, the electrical cross-talk or coupling effect may reach a significant level. As a consequence, the major technical challenge in this approach may be how to suppress the electrical cross-talk and/or coupling effect.
Another approach to improve image quality may be increasing the contrast ratio and/or controlling the contrast grey scale in a precise manner. To display an image, a typical LCD device may split the time domain into a number of frames or intervals. Then, the polarity of voltage applied to each pixel may alternate at the frames, wherein the magnitude of the voltage determines the grey level of the pixel's image. By way of example, a red color may be displayed in 10-bit resolution at the peak-to-peak voltage Vpp of 10 volts. Then, the voltage applied to a pixel may have a resolution of 0.0049 (=10/210) volts in the grey scale. Thus, if the circuit elements have a voltage leak of few milli-volts, the intended red color may not be generated, i.e., a color degraded toward the white may be displayed. As one of the major sources for the voltage leak may be the cross-talk between two neighboring circuit elements and/or cell-to-cell cross talk, the major challenge of this approach would be also how to reduce the electrical cross-talk and/or coupling effect.
The semiconductor chip portion of an LCoS device may have another source of electrical noise: stray light. The stray light noise may be induced by light unintentionally entered into the chip. The stray light may generate electron and hole pairs that are typically converted into electrical noise, which in turn produces the similar effect as the cross-talk and/or coupling.
In view of the above, it would be desirable to design a circuit with reduced electrical noise. Moreover, as the pixel memory capacity for commercial display devices expands at a considerable rate and, as a consequence, each pixel size may decrease rapidly, there is a strong need for an LCoS chip layout that suppresses the electrical noise.